1. Field of the Invention
This invention relates to plasma enhanced, semiconductor substrate processing systems and, more specifically, to a method and apparatus for shaping a magnetic field in a magnetically enhanced plasma reactor.
2. Description of the Related Art
Semiconductor wafer processing chambers commonly employ plasmas to enhance the performance of various processes for fabricating semiconductor devices on silicon substrates or other workpieces. Such processes include sputter etching, plasma enhanced chemical etching, plasma enhanced chemical vapor deposition, and ionized sputter deposition. The high energy level of reagents in the plasma generally increases the rate of the fabrication process, and also reduces the temperature at which the semiconductor workpiece must be maintained to perform the process.
Magnetically enhanced plasma chambers (also referred to as reactors) employ magnetic fields to increase the density of charged particles in the plasma to further increase the rate of the plasma enhanced fabrication process. Increasing the process rate is highly advantageous because the cost of fabricating semiconductor devices is proportional to the time required for fabrication.
Despite this advantage, many plasma chambers in commercial use do not employ magnetic enhancement because the magnetic enhancement has been found to increase the likelihood of damaging the semiconductor devices on the wafer. Such damage is caused by non-uniform electron density across a wafer due to the spatial contour of the magnetic field being poorly optimized.
FIG. 1 depicts a schematic cross section view of a magnetically enhanced plasma chamber 5 suitable for either etching or chemical vapor deposition in accordance with the prior art. FIG. 2 depicts a top cross sectional view of the chamber 5. The chamber 5 is enclosed by an octagonal sidewall 12, circular bottom wall 14 and circular top wall or lid 16. The lid 16 and bottom wall 14 may be either dielectric or metal. An electrically grounded anode electrode 18 is mounted at the bottom of the lid 16. The anode electrode may be perforated to function as a gas inlet through which process gas enters the chamber. The side wall 12 may be either dielectric or metal. If it is metal, the metal must be nonmagnetic material such as anodized aluminum so as to not interfere with the magnetic field created by an array of electromagnetic coils 6, 7, 8, and 9 located outside the chamber 5. If the side wall is metal, it will function as part of the anode.
The semiconductor wafer or workpiece 20 is mounted on a cathode electrode 22, which, in turn, is mounted in the lower end of the chamber 5. A vacuum pump, not shown, exhausts gases from the chamber 5 through an exhaust manifold 23 and maintains the total gas pressure in the chamber 5 at a level low enough to facilitate creation of a plasma, typically in the range of 10 millitorr to 20 torr, with pressure at the lower and higher end of the range being typical for etching or CVD processes, respectively.
An RF power supply 24 is connected to the cathode pedestal 22 through a series coupling capacitor 26 or matching circuit (not shown). The RF power supply 24 provides an RF voltage between the cathode pedestal 22 and the grounded anode electrode 18 that excites the gases within the chamber into a plasma state. The plasma body has a time average positive DC potential or voltage relative to the cathode or anode electrodes that accelerates ionized process gas constituents to bombard the cathode and anode electrodes.
Magnetic enhancement of the plasma most commonly is implemented by a DC magnetic field in the region between the cathode and anode electrodes. The direction of the magnetic field is usually transverse to the longitudinal axis of the chamber 5, i.e., transverse to the axis extending between the cathode and anode electrodes. Various arrangements of permanent magnets or electromagnets are conventionally used to provide such a transverse magnetic field. One such arrangement is the pair of coils 6, 7 shown in FIG. 1 disposed on opposite sides of the cylindrical chamber side wall 12. FIG. 2 depicts a top, cross-sectional view of the chamber of FIG. 1 that shows the orientation of opposing coil pairs 6, 7, 8 and 9. Generally, the diameter of each coil approximately equals the spacing between the two coils. Each pair of opposing coils 6, 7, 8 and 9 are connected in series and in phase to a DC power supply, not shown, so that they produce transverse magnetic fields which are additive in the region between the coil pairs. This transverse magnetic field is represented in FIGS. 1 and 2 by the vector B oriented along the negative X axis. An example of such a magnetically enhanced plasma chamber is described in commonly assigned U.S. Pat. No. 5,215,619, issued Jun. 1, 1993, which is hereby incorporated by reference in its entirety.
Because the plasma has a positive time average potential or voltage relative to the cathode electrode 22, the time average electric field E in the plasma pre-sheath adjacent the cathode is directed downward from the plasma toward the cathode, thereby giving the free electrons in the pre-sheath a drift velocity vector whose time average values oriented upward towards the plasma body, as represented by vector Ve in FIG. 1. In response to the DC magnetic field vector B, these free electrons will primarily experience a qvxB force, causing the electrons and ions to move in a helical shaped path that generally follows the magnetic field vector. In additional, the electrons and ions will experience another time-averaged force due to the combination of the helical motion and the electric field. This is commonly called the ExB drift, where the direction of the drift is approximately coplanar with the semiconductor wafer 20 and orthogonal to the magnetic field vector B as illustrated in FIG. 2 by the ExB vector oriented along the Y axis.
In this discussion, the term “time average” means averaged over one period of the RF frequency or frequencies at which the plasma is excited, this period typically being less than 10−7 seconds. This time average over one RF period is unrelated to the time averaging due to the optional rotation of the magnetic field relative to the workpiece that typically has a rotation period on the order of 0.2 to 4 seconds. The frequency of the electron moving helically about the magnetic field vector is f=(qB)/2 πm, where q is the electron charge, B is the magnetic field strength (Gauss), and f is the frequency (Hertz). For example, a magnetic field of 35 G will result in one turn around the helix lasting about 10e−4 seconds. This is longer than the RF frequency, but is much shorter than the magnetic field rotation of 0.2 to 4 seconds.
It is believed that the ExB drift of free electrons is a major source of semiconductor device damage in conventional magnetically enhanced plasma chambers. Specifically, it is believed that ExB drift can unevenly distribute the free electrons in the plasma pre-sheath and cause non-uniformity in the ion flux. It is believed that this spatial non-uniformity of the ion flux that bombards the wafer produces electrical currents in the wafer which often damages the semiconductor devices on the wafer.
Conventional magnetically enhanced plasma chambers attempt to ameliorate this non-uniformity by slowly rotating the magnetic field relative to the wafer, typically at a rotation frequency in the range of one quarter to five rotations per second. In some designs, the wafer 20 or the magnets 6, 7, 8 and 9 are physically rotated. In other designs, as illustrated in FIG. 2, the rotation is performed electronically by providing two pairs of coils 6, 7 and 8, 9 that are arranged orthogonally to one another. The magnetic field can be rotated in 90° increments by successively and periodically connecting the DC power supply to the first coil pair 6, 7 with positive polarity (2) to the second coil pair 8, 9 with positive polarity; (3) to the first coil pair 6, 7 with negative polarity; and (4) to the second coil pair 8, 9 with negative polarity. Alternatively, the magnetic field can be rotated continuously by replacing the DC power supply with a very low frequency (in the range of 0.1-10 Hz) power supply having quadrature outputs connected to provide current to the first coil pair 6, 7 offset in phase by 90° from the current provided in the second coil pair 8, 9.
Rotating the magnetic field relative to the wafer greatly reduces the time average spatial non-uniformity in the ion flux bombarding the wafer, and therefore can provide acceptable spatial uniformity of etch rate (in an etching chamber) or deposition rate (in a CVD chamber) on the wafer surface. However, rotating the magnetic field does not in any way improve the instantaneous spatial uniformity of ion flux on the wafer surface, and therefore does not completely solve the problem of semiconductor device damage in magnetically enhanced plasma chambers.
U.S. Pat. No. 6,113,731, issued Sep. 5, 2000, discloses a method and apparatus that further combats the ExB drift problem by driving current through the adjacent coil pairs 6, 9 and 7, 8 such that a magnetic field gradient is generated laterally across the surface of the wafer.
In FIG. 2, the magnetic field produced by driving a first current through coils 7, 8 is represented by arrow 10 and the magnetic field produced by driving a second current through coils 6, 9 is represented by the arrows 11. The first current is less than the second current such that the magnetic field 10 is smaller than magnetic field 11 such that a magnetic field gradient is produced, i.e., the magnetic field is shaped. The ratio of the currents produces the specific shape of the gradient. This ratio is optimized for each process regime to create a nearly uniform plasma. For most process regimens, the current ratio is in the range 0.1 to 0.7. This non-uniform magnetic field produces a more uniform ion flux within the chamber by increasing the magnetic field magnitude in the region of the wafer formerly with low etch rate, and by decreasing the magnetic field magnitude in the region of the wafer formerly with high etch rate. This magnetic field gradient is then adjusted to the shape that optimizes ion flux uniformity for each process condition. The optimum magnetic field gradient is dependent upon the hardware configuration used to produce the magnetic fields. As smaller and smaller feature sizes are used on wafers, the requirements for producing a nearly uniform ion flux continue to become more stringent, especially in certain process regimes, in order to prevent damage to the electrical circuitry formed on the wafer. The optimal gradient may be produced in a static position; however when the current is switched to the next coil pair to cause rotation of the plasma, the plasma “jumps” by 90°. Such a “jump” forms a discontinuity in the plasma process that can damage the substrate or cause non-uniform processing.
Therefore, there is a need in the art for a method and apparatus for controlling the magnetic field gradient within a magnetically enhanced plasma chamber.